Triplet Sensitization Photon Upconversion Using Near-Infrared Indirect-Bandgap AgBiS₂ Nanocrystals

Colloidal semiconductor nanocrystals (NCs) have recently emerged as ideal triplet sensitizers owing to their diverse material composition and spectral tunability. However, the NCs that can efficiently sensitize near-infrared-to-visible photon upconversion remain largely limited to toxic lead-based NCs. Here, we present a new lead-free, near-infrared, indirect-bandgap sensitizer based on AgBiS₂ NCs, enabling near-infrared-to-yellow upconversion with a quantum yield reaching 10.5% (normalized to 100%). The key to success is the precise stoichiometry control of AgBiS₂ NCs, which provides the essential surface states for both radiative recombination and triplet energy transfer. Ultrafast transient absorption spectroscopy verifies the efficient triplet energy transfer mechanism mediated by surface states. Our work presents a new eco-friendly material system for efficient triplet fusion near-infrared photon upconversion.